Although prior techniques exist in the art for producing thin lines suitable for forming electrical circuits such as PCBs, many of these techniques suffer from a number of significant disadvantages. For example, many previous techniques suffer from poor resolution. Moreover, techniques which do provide high resolution usually require complex apparatus. A further problem is that previous techniques have required the use of dry films of photopolymer which are usually supported on a polyester (e.g. Mylar) film. The thickness of these dry films has a detrimental effect on the resolution and/or definition of photoimaged surfaces as this allows unwanted undercutting (i.e. light shadowing) to occur during the photoimaging process. There are also problems in adhering partially cured dry films to substrates and contamination problems which once again causes problems in the photoimaging process. Dry films are also expensive when used in large quantities. Such systems are described in U.S. Pat. No. 4,888,270 and U.S. Pat. No. 4,954,421, which are incorporated herein by reference.
At the present time the market for printed circuit imaging may be identified as having two separate types of resist:
(1) Wet resist which is coated on a panel by a variety of means and then pre-dried with hot air to drive off the solvents. This leaves a ‘dry’ surface which is photo-imageable using UV light. The raw materials for wet resist are inexpensive but processing costs (heat etc) substantially add to the overall cost of using a wet resist.
(2) Dry film resist which starts out as a liquid coating that is pre-dried and supplied sandwiched between two layers of protective film. The user laminates the dry film to a copper panel using heat and pressure. In the
process the protective films are removed which requires labour and presents a landfill issue at disposal.
In the prior art both wet resist and dry film resist are exposed using UV light, either by photolithography or by laser direct imaging (LDI).
Today there is an increasing use of UV cured inks in the printing industry. This is because the avoidance of solvents is attractive from an environmental standpoint. It has been customary to use high powered lamps such as mercury discharge lamps to cure the wet ink immediately after it is printed on a moving web of material. However, such lamps consume large amounts of electrical power, generate significant amounts of unwanted heat and also create ozone which needs extraction. Although these types of lamps consume large amounts of power the amount of usable UV energy is a small percentage of the overall output.
By contrast, UV LED's have a much higher output percentage of UV compared to mercury halide filled tubes. They do not create ozone and heat dissipation is minimal and they do not require large bulky ducting. In addition, since the mercury halide lamps do not re-strike immediately when they are switched off then back on again, they have to be left running on partial power even though the line may have stopped for some reason. This involves further heat management as the output from the mercury halide lamp has to be contained usually with shutters which get very hot.
Again, using LED's which have more or less instant start up saves wasted power on standby and overall electrical power used with LED's is a fraction of that used with conventional mercury halide lamps.
The one weakness of LED's is that they produce significantly lower levels of overall UV power albeit they are efficient at what they do. This means that a printer seeking to use LED's for curing will typically have to run their line slower to allow longer time for the ink to cure.
One reason for slow cures is a phenomenon known as oxygen inhibition whereby the presence of air at the cured surface interferes with the tendency for the ink to cure under UV light. To get round this, some printers create an atmosphere of inert gas (e.g. nitrogen) in the curing area which effectively prevents oxygen from interfering with the process. This is an expensive means to an end.
We also hereby refer to WO2010/007405, incorporated herein by reference, which refers to using a resist that is different to pre-dated prior art resists in that it is made up of 100% solids so has no solvents involved in the related processing. In this process, the ink is coated on a panel but is not pre-dried prior to imaging but is sandwiched under a layer of clear film. During exposure to UV light the resist hardens in the exposed areas only. After imaging, the protective polyester is peeled off for re-use and the unexposed (liquid) resist is washed off the panel. The resist in WO2010/007405 can also be exposed using either photolithography or laser direct imaging (LDI). WO2010/007405 also solely relates to using a fixed and non-rotatable phototool.
The outside surface of a printed circuit board (PCB) is usually covered with a protective insulative layer which assists soldering by confining it to specific areas such as electrical components or pads. As components and circuitry size decreases the need for accurate registration of the solder mask is becoming increasingly important. The need is to apply the same imaging accuracy using laser direct imaging (LDI). However, the solder mask can be relatively thick (e.g. 75 microns) so exposure using a laser is a slow job. As lasers are expensive to buy and operate it is a major commercial decision for a manufacturing company to make this extra investment.
We also hereby refer to WO2010/007405, incorporated herein by reference, which refers to using a photopolymer that is different to pre-dated methods in that it is made up of 100% solids so has no solvents involved in the related processing. In WO2010/007405, the ink is coated onto a panel but is not pre-dried prior to imaging and is sandwiched under a layer of polyester or other UV transparent material. During exposure to UV light the photopolymer hardens in the exposed areas only. After imaging, the protective polyester is peeled off for re-use and the unexposed (liquid) photopolymer is washed off. The photopolymer in WO2010/007405 can also be exposed using either photolithography or laser direct imaging (LDI). WO2010/007405 also solely relates to the exposure of the photopolymer using a phototool.
It is an object of at least one aspect of the present invention to obviate or mitigate at least one or more of the aforementioned problems.
It is a further object of at least one aspect of the present invention to provide an improved method for photoimaging surfaces.
It is a yet further object of at least one aspect of the present invention to provide a cost efficient method for producing electrical circuits with high resolution and small track widths (i.e. fine lines).
It is a further object of at least one aspect of the present invention to provide a cost efficient method for producing high density electrical circuits suitable for PCBs, flat panel displays and flexible circuits.
It is a further object of at least one aspect of the present invention to provide an improved method for photoimaging surfaces with high resolution and small track widths over a large area.
It is a further object of at least one aspect of the present invention to provide an improved method for exposing at least part of a solder mask.